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2.3.3The Particulate Nature of Matter

Properties of Simple Molecular Compounds — Combined Science 0653 - Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 11 of 13 Unit quiz

📚 Combined Science 0653 - Chemistry Hub / Topical Revision
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Part 1 of 4Properties of small molecules

Low melting and boiling points
  • Low melting and boiling points Simple molecular substances melt and boil at low temperatures.
  • This is why most are liquids or gases at room temperature.
Size matters
  • Size matters As the molecules get larger, the melting and boiling points generally increase.
Poor conductors
  • Poor conductors They do not conduct electricity, so they are insulators.

Part 2 of 4Explaining the melting and boiling points (Extended)

1Two kinds of attraction
  • Two kinds of attraction Strong covalent bonds hold the atoms together within each molecule.
    • Weak intermolecular forces act between neighbouring molecules.
2Melting or boiling
  • Melting or boiling Only the weak intermolecular forces are overcome when the substance melts or boils.
    • The covalent bonds stay intact.
3Little energy needed
  • Little energy needed Because these forces are weak, little energy is required, so the melting and boiling points are low.
4Larger molecules
  • Larger molecules Bigger molecules have more electrons, so the intermolecular forces are stronger and more energy is needed.

Part 3 of 4Covalent bonds compared with intermolecular forces

FeatureCovalent bondIntermolecular force
Where it actsBetween atoms inside a moleculeBetween separate molecules
Relative strengthVery strongVery weak, roughly a tenth as strong
Broken whenChemical reactions occurMelting and boiling
Example in waterO-H bond within the moleculeAttraction between neighbouring H2O molecules

Part 4 of 4Electrical conductivity (Extended)

1
To conduct electricity, a substance needs charged particles that are free to move, such as ions or delocalised electrons.
2
Simple molecules are neutral, because their electrons are held within covalent bonds.
3
There are no free ions or electrons, so charge cannot flow through the substance.
4
Most molecular compounds therefore do not conduct, even when solid or liquid.
5
This is why the plastic coating on household wires, which is made of covalent molecules, is a good insulator.

Key terms

TermMeaning
Simple moleculeA small particle made of a few atoms joined by covalent bonds.
Intermolecular forceA weak attraction between neighbouring molecules.
InsulatorA material that does not allow electric charge to flow through it.
Melting pointThe temperature at which a solid turns into a liquid.
Boiling pointThe temperature at which a liquid turns into a gas.

Avoid these mistakes

1
Writing that covalent bonds break when a simple molecular substance melts or boils. Only the intermolecular forces are overcome.
2
Saying that the covalent bonds are weak. They are strong; the forces between molecules are weak.
3
Explaining the lack of conductivity as no electrons at all. The electrons are present but not free to move.
4
Forgetting that larger molecules have higher melting and boiling points.

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